9.101 FLUSH — Flush I/O unit(s)

Description:

Flushes Fortran unit(s) currently open for output. Without the optional argument, all units are flushed, otherwise just the unit specified.

Standard:

GNU extension

Class:

Subroutine

Syntax:

CALL FLUSH(UNIT)

Arguments:

UNIT	(Optional) The type shall be INTEGER.

Note:

Beginning with the Fortran 2003 standard, there is a FLUSH statement that should be preferred over the FLUSH intrinsic.

The FLUSH intrinsic and the Fortran 2003 FLUSH statement have identical effect: they flush the runtime library's I/O buffer so that the data becomes visible to other processes. This does not guarantee that the data is committed to disk.

On POSIX systems, you can request that all data is transferred to the storage device by calling the fsync function, with the POSIX file descriptor of the I/O unit as argument (retrieved with GNU intrinsic FNUM). The following example shows how:

! Declare the interface for POSIX fsync function
interface
  function fsync (fd) bind(c,name="fsync")
  use iso_c_binding, only: c_int
    integer(c_int), value :: fd
    integer(c_int) :: fsync
  end function fsync
end interface
          
! Variable declaration
integer :: ret
          
! Opening unit 10
open (10,file="foo")
          
! ...
! Perform I/O on unit 10
! ...
          
! Flush and sync
flush(10)
ret = fsync(fnum(10))
          
! Handle possible error
if (ret /= 0) stop "Error calling FSYNC"